Most desktop computers nowadays use power supplies of ATX specifications. The power supply that adopts the ATX specifications has a main feature, i.e. including a primary power system and a standby power system. After the power supply has received commercial power, the standby power system automatically generates a standby power supplying to other systems (such as primary power system, protection system and control chip sets and the like) to enter standby conditions. When users start the power supply (usually at the same time of starting the computer, the start signal starting the power supply first), then the primary power system starts to transform the commercial power to primary power to supply other devices in the computer.
Refer to FIG. 1 for the fundamental circuit architecture of a conventional power supply. The power supply has a first rectification unit 2 to receive commercial power from a commercial power output end 1 and rectify the commercial power, a power factor correction unit 3 to modulate commercial power factor, a primary power system 4 and a standby power system 5 that respectively connect to the power factor correction unit 3. After receiving the power, the standby power system 5 which includes a first transformer 51, a first switch 52, a first pulse modulation unit 53 and a second rectification unit 54, generates a standby power 55. The primary power system 4 and a power management unit 6 receive the standby power 55 and enter a standby condition to be started by users. When an user provides a start signal 7 (PS_ON) to the power management unit 6, the power management unit 6 outputs an enabling signal 71 to a second pulse modulation unit 43 in the primary power system 4 to start operation, the primary power system 4 has a second transformer 41 and a second switch 42 under the control of the second pulse modulation unit 43 to transform power, and through a third rectification unit 44, a primary output power 45 at 12V is formed. The primary output power 45 of 12V also can be transformed through at least one conversion unit 46 to become other forms of primary output power 45 at 5V or 3.3V to be used by different devices. The conversion unit 46 may include a plurality of converters. The technique for the primary power system 4 to provide output power at multiple voltages is known in the art, details are omitted here.
As previously discussed, the primary power system 4 and standby power system 5 have different starting conditions and time series. In the conventional power supply, the primary power system 4 and standby power system 5 are separated and independent. Each has its own transformer and related control circuit to generate independent output power. Hence more circuit elements are needed to form the primary power system 4 and standby power system 5. As a result, it is difficult to shrink the size and lower the cost.